The present invention relates generally to measurement probes and more specifically to a replaceable probe tip holder for a wide bandwidth, high frequency measurement probe, such as used with measurement test equipment like oscilloscopes, logic analyzers and the like.
A typical measurement probe is described in U.S. Pat. No. 5,061,892 for acquiring electrical signals from a device under test in the megahertz range. The probe has probe head with an electrically conductive hollow tube having a substrate disposed therein. The substrate has passive and/or active circuitry to prevent loading of the test device. The end of the hollow tube has an insulating plug disposed therein with a coaxially disposed probing tip extending out of the plug in both directions. The portion of the probing tip extending into the hollow body is electrically connected to the substrate by a spring loaded electrical contact electrically connected to the substrate. A damaged or broken probe tip may be replaced by replacing the plug and associated probe tip. One drawback to this type of probe design is limited bandwidth of the probe caused by the capacitance of the probe tip and the spring loaded electrical contact. At high frequencies, measures need to be taken to minimize the probe tip capacitance.
High frequency measurement probes, such as for probing signals in the 3 Ghz to 4 Ghz range, require high input resistance values and low input capacitances. Generally, these types of probes have an active FET input for low circuit loading and have a frequency bandwidth up to 4 Ghz or more. An example of such a probe is the P6217 Active FET Probe, manufactured and sold by Tektronix, Inc., Beaverton, Oreg. and assignee of the instant application.
FIG. 1 is a cross-sectional view of the P6217 probe 10 showing the internal configuration of the probe elements. The probe 10 has an electrically conductive circular tubular housing 12 in which is disposed a substrate 14. The substrate is slightly off center in the tubular housing so that the upper surface of the substrate is centered in the housing. The end of the substrate 14 is tapered to provide clearance for a probe tip holder 16 and extends slightly outward from the end of the tubular housing approximately 0.025 inches. Passive and active electrical components 18, such as FET transistors, resistors, capacitors and the like are mounted on the substrate. A gold foil contact 20 extends over the front end of the substrate 14 for providing an electrical connection between the substrate 14 and a probing tip 22. The probe tip holder 16 is made of insulating material, such as plastic or the like, is secured within the end of the housing 12. The holder 16 has a cavity 24 with a tapered end 26 that receives a portion of the substrate 14 extending out past the end of the housing 12. The tapered end 26 is centrally formed in the holder 16 and has a bore 28 extending therefrom to the front end of the holder 16. The probing tip 22 has a head 30 and a shank 32 that is disposed in the holder 22 with the head 30 positioned in the tapered end 26 and the shank 32 extending through the bore 28 with the tip of the shank 32 exposed at the front end of the holder 16. A resilient elastomeric member 34 is disposed in the tapered end 26 between the probing tip head 30 and the holder to provide a compressive force between the head 30 and the gold foil contact 20 on the end of the substrate 14. An insulating boot 36 is positioned over a substantial portion of the tubular housing 12 to electrically insulate the housing 12 from the user.
A major drawback to the probe shown in FIG. 1 is that the probing tip 22 is not replaceable. FIG. 2 shows the structure of the holder 16 that is secured into the tubular housing 12. The holder 16 has an outer flange 38 that abuts against the tubular housing 12 when the holder 16 is inserted into the housing. Extending from the flange 38 is a circular support member 40 that is received in the tubular housing 12. Because of the position of the substrate 14 within the housing 12, the lower portion of the support member 40 is removed to provide clearance for the holder 16. To allow the holder 16 to be inserted into the housing 12 in two positions, the upper portion of the support member 40 is also is also removed. This weakens the support member 40. To increase the strength of the support member 40, the rounded inner surfaces are thickened to produce opposing approximate half-mooned shaped support members 42 having parallel surfaces 44. The resulting shape of the support member 40 requires precise alignment of the holder 16 in the tubular housing 12. The parallel surfaces 44 need to be positioned perpendicular to the top surface of the substrate 14. Positioning the parallel surface 44 in a non-vertical position causes the support member 40 to hit the substrate 14 causing irreparable damage. Precisely aligning the holder 16 in the tubular housing 12 requires the use of a microscope and special fixturing that is not available to a user. Not being able to replace broken probing tips on such probes becomes expensive for customers with such probes costing in the $3,000 range.
What is needed is a replaceable probe tip holder and an associated probe head for a measurement probe. The probe tip holder should be easily replaceable without a need for special tools, fixturing or microscopes. The holder and associated probe head configuration should minimize the probe tip capacitance to provide a wide bandwidth, high frequency measurement probe.
Accordingly, the present invention is a replaceable probe tip holder for a measurement probe head. The probe tip holder includes a cap and attachment arms extending from the back end of the cap that are positionable on the outside of a tubular housing of the probe head. The back end of the cap has a first cavity formed therein that is sized to receive a portion of the front end of the tubular housing. A second cavity extends from the first cavity and is sized to receive a portion of a substrate that extends beyond the front end of the housing. At least a first bore is formed in the cap from the second cavity to the front end of the cap. Each attachment arm may include a latching member in the form of a protrusion or an aperture. The protrusions are formed on inward facing surfaces of the attachment arms and are circular in form with each protrusion having an angled surface extending from the surface of the attachment arm to a top surface of the protrusion. The probe tip holder may also be configured with a second bore formed from the second cavity to the front end of the cap and disposed adjacent to the first bore. At least a first probe tip cavity may also be formed in the cap extending from the second cavity and sized to receive a first resilient compression member with the first bore formed from the probe tip cavity to the front end of the cap. A second probe tip cavity may also be formed in the cap that is disposed adjacent to the first probe tip cavity and extends from the second cavity with a second bore formed from the second probe tip cavity to the front end of the cap. The probe tip cavity is sized to receive a second resilient compression member. In the preferred embodiment of the present invention the center to center separation of the first and second bores is in the range of 0.100 inches. The depth of the first cavity in the range of 0.165 inches and the depth of the second cavity is in the range of 0.100 inches. The resilient compression member is preferably a nonconductive elastomer. A first electrically conductive probe tip having a shank and a head with the shank is positioned within the cap such that the head is in contact with the first resilient compression member and the shank extends through the first bore with the end of the shank extending from the front end of the cap. A second electrically conductive probe tip configured the same as the first probe tip is positioned within the cap such that the head is in contact with the second resilient compression member and the shank extends through the second bore with the end of the shank extending from the front end of the cap.
A high frequency measurement probe head incorporating the replaceable probe tip holder has an elongate tubular housing with a front end and a back end. A substrate is disposed in the housing with a portion of the substrate extending beyond the front end of the housing. The portion of the substrate extending beyond the front end of the housing has a front surface with at least a first electrically conductive contact disposed thereon. The probe tip holder has a cap with a front end and a back end. A first cavity formed in back end of the cap is sized to receive a portion of the front end of the housing. A second cavity extends from the first cavity and is sized to receive the portion of the substrate extending beyond the front end of the housing. At least a first probe tip cavity extends from the second cavity with a bore formed from the probe tip cavity to the front end of the cap. At least a first resilient compression member is disposed in the first probe tip cavity. At least a first electrically conductive probe tip having a shank and a head is positioned within the cap such that the head is in contact with the first resilient compression member and the shank extends through the bore with the end of the shank extending from the front end of the cap and the probe tip head making electrical contact with the first electrically conductive contact on the substrate. The holder includes attachment arms extending from the back end of the cap that are positionable on the outside of the housing.
The probe head may further include a latching arrangement having at least a first protrusion member and an aperture member capable of receiving the protrusion member. One of the protrusion member and aperture member is formed in each of the attachment arms and the other of the protrusion member and aperture member is formed on opposing sides in the elongate tubular housing. The protrusion member is preferably circular in form and has a base and top surface and an angled surface extending from the base to the top surface. The tubular housing is preferably rectangular in shape having recessed channels formed on opposing sides of the rectangular shaped tubular housing. The latching arrangement has one of the protrusion member and aperture member formed in each of the attachment arms and the other of the protrusion member and aperture member formed in each of the recessed channels of the elongate tubular housing.
The probe head may further include a second probe tip cavity extending from the second cavity and disposed adjacent to the first probe tip cavity with a bore formed from the second probe tip cavity to the front end of the cap. A second resilient compression member is disposed in the second probe tip cavity and a second electrically conductive probe tip having a shank and a head is positioned within the cap such that the head is in contact with the second resilient compression member and the shank extends through the second bore with the end of the shank extending from the front end of the cap. The respective resilient compression members are preferably formed from a nonconductive elastomer. The first and second electrically conductive probe tips are separated by a distance in the range of 0.100 inches. The first cavity has a depth in the range of 0.165 inches and the second cavity has a depth in the range of 0.100 inches. The front end surface of the substrate extends beyond the front end of the housing in a range of 0.100 inches.
The objects, advantages and novel features of the present invention are apparent from the following detailed description when read in conjunction with appended claims and attached drawings.